An important goal of the hard disk drive industry is to develop magnetic heads for hard disk drives that provide ever faster data writing speeds, and that allow data to be written with ever increasing areal data storage density, that is, with more bits of data written per unit of disk area.
As is known to those skilled in the art, prior art magnetic head designs include a GMR sensor having a plurality of thin film layers that include a free magnetic layer and a bias layer. As the magnetic head passes over various data bit positions along a data track on the disk, the magnetic field of the free magnetic layer rotates in response to the magnetic fields of the data bits recorded on the hard disk. The bias layer has relatively high magnetic coercivity and acts as a hard or permanent magnet to provide a bias magnetic field that biases the free magnetic layer. Under the influence of this bias magnetic field, the magnetic fields of the free magnetic layer are generally stable and oriented in a single direction.
A problem arises in using a bias layer in a read head designed for a narrow track width, specifically, the magnetic domains within the bias layer can become unstable due to thermal and other stresses. When these effects occur the bias layer tends to loose its ability to stabilize the free magnetic layer.